A general example of a fabric filter system is disclosed e.g. in U.S. Pat. No. 4,336,035. Such a filter may comprise a plurality of filter bags which separate particulate matter from a gas flow. A typical application of such a filter system is to clean flue gases from a coal combustion process.
In some cases it is desired to substantially stop the gas flow through the filter system, e.g. in order to allow service staff to enter one filter system interior, while filtering is continued by other, parallel filter systems.
This may be provided by means of a flow control device of the initially mentioned kind which is then accomplished by means of a flap damper, which may be devised as a hinged door which is pivotably moveable between two positions, one where the opening to the duct is open, and one where the opening is covered by the door.
One problem associated with such a filter system is that the door may become difficult to operate, particularly if the cross section of the opening is large, such that a large door is to be used. A considerable force is needed e.g. to open the damper.